In today's society where many electronic devices are used, various kinds of data are being produced and used. In order to store such data, it is necessary to use a memory element (hereinafter also referred to as “memory”). Various kinds of memory that are produced and used now have both advantages and disadvantages and are selectively used according to the kind of data to be used and stored therein.
Memory can be broadly divided into two kinds, i.e., volatile memory and nonvolatile memory. Volatile memory is memory whose memory content is lost once power is turned off. Nonvolatile memory is memory that can retain its memory content even when power is turned off. Examples of volatile memory include DRAM (dynamic random access memory) and SRAM (static random access memory). The use of volatile memory is very limited as the memory content is lost once power is turned off; however, the volatile memory is used for cache memory of computers or the like because of its advantage of a short access time. DRAM has small memory cells and can have a high capacity; however, a method of controlling the DRAM is complex, resulting in high power consumption. Meanwhile, methods of fabricating and controlling SRAM are easy since it has memory cells constructed from CMOS; however, the SRAM has a difficulty in having a high capacity since each memory cell requires six transistors.
Nonvolatile memory, which is memory capable of retaining its memory content even when power is turned off, can be broadly divided into three kinds, i.e., rewritable memory, write-once memory, and mask ROM (read only memory). Rewritable memory can be rewritten with data up to a limited number of times. Write-once memory can be written with data by a user only once. The data content of mask ROM is determined during its fabrication, and the data content cannot be rewritten.
Examples of rewritable nonvolatile memory include EPROM, flash memory, and ferroelectric memory, EPROM can be written with data easily, and has a relatively low unit cost per bit. However, EPROM requires dedicated programming and erasing devices for writing and erasing data. Meanwhile, flash memory and ferroelectric memory can be rewritten with data on a substrate being used, and have a short access time and low power consumption.
As an example of a structure of flash memory, there is known a structure in which a tunneling insulating film, a floating gate, a gate insulating film, and a control gate are formed over an active layer (see Reference 1: Japanese Published Patent Application No. 2006-13481). In such nonvolatile memory of a floating-gate type, electrical charge is injected to the floating gate through the tunneling insulating film over the channel formation region which is formed in the active layer, so that the electrical charge is retained in the memory.